MECHANICS OF FRETTING FATIGUE-CRACK FORMATION

Fretting is a contact damage process arising from surface microslip as sociated with small-scale oscillatory motion of clamped structural mem bers. The fretting damage process is a synergistic competition among w ear, corrosive and fatigue phenomena driven by both the microslip at t he contact surface and cyclic fretting contact stresses. Fretting fati gue is one mechanism of the formation of cracks in many common structu ral members, often leading to multi-site damage in riveted lap joint a ssemblies in aging aircraft. Thus a criterion for prediction of fretti ng fatigue crack nucleation is needed. A detailed analysis of the micr oslip distribution at the contact surface and the subsurface stress fi eld is required for such a prediction. Relevant closed-form solutions for the 2-D elastic stress fields are adapted for reduced loading conf igurations modeled in a recently constructed fretting fatigue experime nt that applies loads relevant to aircraft lap joints. The resulting s tress field is combined with a multiaxial fatigue theory that combines strain-life ideas with a maximum normal stress to predict both the in itiation site and life of fretting fatigue cracks. In particular, the theory predicts formation at the trailing edge of contact-not the loca tion of the maximum shear stress traditionally associated with crack f ormation in contact fatigue. The fretting fatigue crack nucleation the ory is validated through comparison with data in the literature. Once validated, the model is used to investigate the effects of coefficient of friction, load intensity and fatigue properties on life. It is sho wn that increases in coefficient of friction and surface microslip sha rply reduce the number of cycles required to nucleate cracks. Applicat ion of the fretting fatigue crack nucleation model to actual loading c onfigurations in common structural members such as riveted lap joints can lead to a tool for evaluating fatigue life of those members.